It's not always that obvious! Our advice is to watch the water reservoir when you are filling your machine and stop below the small (removable) grid that you can see on the surface. It is important not to add more water because the 16 bars stored in the machine cannot push through more than 50ml of water.
When using ground coffee, just remove the grid (which serves as an indicator). Thus, you will have to stop pouring water at the level of the crosspiece supporting the ground coffee adapter.